Large generators and motors are routinely checked when stationary for interlaminar short circuits between the laminates, which are normally isolated from one another, in the laminated core of the stator. However, it is desirable to detect faults such as these during operation of the machine since this allows interlaminar short circuits to be identified earlier and allows measures to prevent further damage to be taken earlier.
US patent specification U.S. Pat. No. 5,032,826 (Miller et al.) discloses the shaft voltage of the machine being measured and being used to draw conclusions about interlaminar short circuits. The shaft voltage is that voltage which occurs along the shaft, in particular between the two ends, or between the shaft and ground during operation of the electrical machine. According to U.S. Pat. No. 5,032,826, changes in the shaft voltage are assessed in the frequency domain or in the time domain, as an indication of interlaminar damage.